In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range. Also the term “channel” shall be used which refers to a discrete wavelength of light in which a distinct set of data is transmitted.
Optical Performance Monitors (OPMs) are used within optical networks for monitoring the optical properties of optical signal channels such as channel powers throughout an optical node or optical system. Their output can be used in controlling the signal powers of optical signal channels in a node. They are particularly useful in Raman amplifiers where it is difficult to control the gain in the fibre since it is an integral part of the optical transmission system, so power control at the Raman laser power injection point in the span for co-propagating pumping is often carried out. OPMs can be used to monitor a single channel, dual channel or multiple channels.
Conventional OPMs are typically made of fused couplers that tap a fixed proportion of light into another fibre and to a photodiode, typically a PIN diode. The PIN diode is used in determining the total light power which could be made up of light from a single channel or multiple channels including noise. This provides useful information, but does not give any granular detail about the signal(s) and noise at the monitor point. In dense wavelength division multiplexed (DWDM) transmission, channels are represented by wavelengths of the optical light. To measure each discrete wavelength of light a filter must be used in front of the PIN diode. Measuring different channels or different groups of channels may be implemented by changing the filter's response to the incident light so that the centre wavelength or bandwidth of the filter is changed. An example filter could be an interference filter made from a stack of optical thin films or an etalon and by changing the temperature of the component the transmission can be modified to allow a selection of specific wavelengths. However, varying temperature to repeatedly measure each channel across a set of DWDM signals is slow and measurement must be carried out in a sequential order. The temperature changes are not instantaneous so the OPM “sweeps” through numerous wavelengths sequentially before the required channel is reached for measurement. In optics, this is a relatively slow and laborious process.
OPMs that read the optical power from a single channel or from multiple channels at an optical amplifier node can also be used to measure optical power at several different positions within the node. This is achieved with a single OPM having multiple input ports, e.g. one for each physical position. Such OPMs conventionally measure a fixed number of channels, in a fixed order, and in a fixed time-frame, which do not change according to the operating conditions of an optical network.
As networks become less point-to-point and more mesh-like, the routes optical channels take to get through a network are less deterministic. Therefore, there is a significant need to flexibly measure and control the individual channels at any point in an optical node or network. This is useful both in a stable condition as well as conditions where the speed of changing the signal wavelength configuration is key to optimising usage of channels passing in nodes or through the network while minimising disruptions caused by this adding/dropping of channels. There is a need within a network having many optical nodes for improving the control and monitoring of optical nodes to enhance and optimise the performance of the optical network.